<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>2focus伪类选择器</title>
  <style>
    input:focus {
      background-color: red;

    }
  </style>
</head>

<body>

  <div>
    <input type="text">
    <input type="text">
    <input type="text">
  </div>

</body>

</html>